1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus, and more particularly, to a surface acoustic wave apparatus in which a SAW chip is accommodated into a package including a multilayer board.
2. Description of the Related Art
In a conventional surface acoustic wave apparatus, a metallic, hermetically sealed case has been used as a packaging member for accommodating a SAW chip. Since a lead terminal extends from the hermetically sealed case, however, the apparatus cannot be surface-mounted.
To allow surface-mounting, a surface acoustic wave apparatus using a multilayer board as a part of its packaging member has been proposed (for example, in Japanese Unexamined Patent Publication No. 4-263509).
In a conventional SAW apparatus in which a SAW filter chip is accommodated into a package, the ground electrodes of input and output inter-digital transducers (IDTs) are connected in common to the ground electrode of the package member in order to simplify a wiring process.
With such a structure, however, sufficient attenuation cannot be obtained at the outside of the pass band. The reason for such a disadvantage will be described below by referring to FIG. 1.
FIG. 1 illustrates an equivalent circuit of a conventional surface acoustic wave apparatus used for describing the effects of inductances created by bonding wires and the ground electrode of the package member. A SAW filter chip 1 is connected between an input end 2 and an output end 3. Between the input end 2 and the SAW filter chip 1, there exists an inductance L.sub.1 created by the input electrode made on the package member and an inductance L.sub.2 created by the bonding wire connected between the input electrode of the package member and the hot-side electrode of the input IDT of the SAW filter chip 1. On the other hand, between the SAW filter chip 1 and the output end 3, there exists an inductance L.sub.3 created by the bonding wire connected between the hot-side electrode of the output IDT of the SAW filter chip 1 and the output electrode disposed on the package member and an inductance L.sub.4 created by the output electrode.
The ground electrodes of the input IDT and the output IDT are connected in common to the same ground pattern of the package member in order to reduce the number of terminals to be mounted and to facilitate a mounting process. In other words, the ground electrode of the input IDT of the SAW filter chip 1 is connected to the ground pattern of the package member with bonding wire. An inductance L.sub.5 caused by this bonding wire is introduced between the ground electrode of the input IDT and the ground pattern of the package member. In the same way, an inductance L.sub.6 is created by bonding wire between the ground electrode of the output IDT and the ground pattern of the package member. An inductance L.sub.7 is created by the ground pattern of the package member.
In the above conventional surface acoustic wave apparatus, sufficiently large attenuation cannot be obtained outside the pass band due to the effects of the inductances L.sub.5 and L.sub.6, caused by the above bonding wires at the ground side, and the inductance L.sub.7, created by the ground pattern of the package member.
In the surface acoustic wave apparatus specified in Japanese Unexamined Patent Publication No. 4-263509, a SAW filter chip is accommodated into a package member using a multilayer board, and the ground electrode of the input IDT and the ground electrode of the output IDT in the SAW filter chip are connected to ground patterns located on different layers of the package member to increase attenuation outside the pass band. In other words, as shown in FIG. 2, the ground electrode of the input IDT and the ground electrode of the output IDT in the SAW filter chip are electrically connected to different ground electrodes located on different layers of the package member in the conventional surface acoustic wave apparatus.
An inductance L.sub.8 created by the bonding wire and a residual inductance L.sub.9 caused by a first ground electrode disposed in the package are introduced between the ground electrode of the input IDT and the ground. An inductance L.sub.10 caused by the bonding wire and an inductance L.sub.11 caused by a second ground electrode located in the package are introduced between the ground electrode of the output IDT and the ground.
As described above, it is noted in the Publication No. 4-263509 that the inductances to the ground are completely separated between the input side and the output side, and thereby a decrease in attenuation outside the pass band is avoided.
A surface acoustic wave apparatus has been provided with advanced functions recently. A surface acoustic wave apparatus is proposed, for example, in which a plurality of SAW filters having different pass-band characteristics are accommodated into one package. In such a structure, when the ground electrodes of the input and output sides of a plurality of SAW filters are individually connected to ground electrodes located in the package, the surface acoustic wave apparatus must be large and cannot have a reduced size.
To reduce the overall dimensions of an entire surface acoustic wave apparatus, very fine electrodes must be formed in order to independently connect the ground electrodes of the input and output sides of each SAW filter chip to ground electrodes located in the package. Thus, a connection process with bonding wire becomes very complicated. In addition, the cost increases significantly and a manufacturing yield is reduced.
As a SAW filter uses higher frequencies, it becomes difficult to suppress noise just by independently connecting the input ground electrodes and the output ground electrodes because of a large effect of residual inductance.
When a number of independent ground electrodes are disposed in a package member, paths for leading the case ground of the package to the outside are limited and sufficient case grounding may not be obtained. Thus, noise may become large in the surface acoustic wave apparatus.